[DAGCombiner] Change the SDLoc on split extloads (2/N)

In DAGCombiner, we try to simplify this pattern:

  ([s|z]ext (load ...))

Conceptually, a new extload which is created while splitting the load
should have the same debug location as the load.

Making this change affects the IROrder of the new load, causing some
test case churn.

In practice, the new location is never different from the location of
the [s|z]ext, at least not during check-llvm or a stage2 build.

Part of: llvm.org/PR37262

Differential Revision: https://reviews.llvm.org/D46156

llvm-svn: 331301